RFA: Recognize 'x' in response to 'p' packet

[Paul Schlie]

> Jim Blandy
> * remote.c (fetch_register_using_p): Recognize 'x's for the value
>   of the register as indicating that the register's value is not
>   available.

Out of curiosity, under what practical circumstances would the value of a
register not be accessible? (and if not, shouldn't an error be returned, as
opposed to an 'x' which is converted to a 0 anyway? Which I've noticed "g"
packets also assume?)